This invention relates to apparatus for either (1) continuously producing photovoltaic devices on a substrate by depositing successive semiconductor layers in each of at least two adjacent deposition chambers through which the substrate continually travels, or (2) utilizing batch processing techniques to produce photovoltaic devices by depositing successive semiconductor layers in each of at least two unconnected deposition chambers into which the substrate is successively transported. Since the composition of the semiconductor layers is dependent upon the particular process gases introduced into each of the deposition chambers, even small amounts of impurities or contaminants in the semiconductor layers deleteriously effect the efficiencies of photovoltaic devices produced. Therefore, process gases introduced into the deposition chamber itself, must be carefully controlled. To that end, the deposition chamber is sealed to the atmosphere, pumped to low pressures, heated to high temperatures and flushed with a gas such as hydrogen or argon prior to initiation of the glow discharge deposition process.
In glow discharge deposition techniques currently employed, the process gases are introduced at spaced intervals along one of the sides of the deposition cathode. The process gases are drawn by a vacuum pump across the deposition surface of a substrate where an r.f. powered cathode or a microwave generator creates an electromagnetic field in the region defined between that deposition cathode or microwave generator and the substrate (hereinafter referred to as the "plasma region" or "deposition region"). The process gases, upon entering the electromagnetic field are disassociated into an ionized plasma which is adapted to be deposited onto the exposed surface of the substrate.
However, it has now been determined that the semiconductor material produced adjacent the upstream section of the substrate, that section of the substrate first contacted as the process gases flow across the deposition surface thereof, exhibits electrically inferior characteristics as compared to the semiconductor material deposited onto the downstream deposition surface of the substrate. The electrically inferior characteristics of the upstream semiconductor material can be attributed, inter alia, to (1) impurities in the process gases intially entering the plasma region of the deposition chamber and more quickly depositing onto the substrate in that chamber, than the desired process gas species, (2) contamination from the ambient conditions existing in said deposition chamber when the process gases first contact the energized electromagnetic field and also more quickly depositing onto the substrate in that chamber than the desired process gas species, and (3) the changing chemical combinations and bonding formations which are formed as the process gases move across and are subjected to the electromagnetic field in each deposition chamber.
More particularly, despite efforts to procure "pure" process gases, at least trace amounts of impurities are present. In prior glow discharge deposition apparatus, these impurities were deposited as the process gases contacted the electromagnetic field at the upstream side of the substrate. Further, despite pumping and cleansing efforts, contaminants would outgas from the walls of the deposition chamber when the deposition cathode or microwave generator was powered to create the electromagnetic field. These impurities and contaminants would be deposited on the upstream side of the substrate, thereby contributing to the electrically, chemically and optically inferior upstream semiconductor material.
It has also been found that the composition of the semiconductor film deposited onto the substrate in such prior deposition apparatus varies with the length of time the process gases are subjected to the effects of the electromagnetic field. In other words, the species and compounds formed when the process gases initially come into contact with and are disassociated by the electromagnetic field vary from the species and compounds deposited onto the substrate at a more downstream location. Although, the precise physical and chemical properties of the species and compounds deposited at the downstream location are currently being investigated and have not as yet been fully identified, it is apparent that they provide superior electrical and optical responses (as compared to the responses of the material deposited at the upstream location).
Whether those improved electrical and optical responses are due to the removal of trace impurities from the process gases, the removal of contaminants outgassed from the walls of the deposition chamber, the formation and breakdown of species and compounds, or a combination of all of the foregoing, it is clear that the properties exhibited by the material deposited onto the substrate is dependent on the length of time the precursor process gases spent in the presence of an electromagnetic field. In other words, the overall electrical, chemical and optical properties of semiconductor devices produced from semiconductor layers deposited onto a substrate are superior at the downstream segment of the substrate.
Accordingly, it is one principle object of the upstream cathode system of the present invention to create an electromagnetic field upstream of the deposition cathode or microwave generator for (1) collecting impurities from the process gases and contaminants from the walls of the deposition chamber and/or (2) subjecting the process gases to a predeposition electromagnetic field prior to their introduction to the deposition electromagnetic field. In this manner, an improved, stable semiconductor film is deposited onto the substrate, said film being of substantially uniform and homogeneous composition across the surface of the substrate and exhibiting improved photovoltaic characteristics.
A second problem encountered by Applicants in the fabrication of amorphous semiconductor material by glow discharge plasma deposition was a limitation imposed on the speed of deposition. Initial deposition rates were limited to approximately 4 to 5 angstroms per second in order to produce material of acceptable quality. In view of the fact that for the mass production of semiconductor material, higher deposition rates would be necessary, Applicants attempted to deposit the amorphous semiconductor material at a rate of 10 to 12 angstroms per second by increasing the power and/or changing the ratios of the precursor reaction gases such as silane and molecular hydrogen. The disappointing results consisted of (1) the formation of powder in the deposited amorphous semiconductor material, and (2) a significant disparity in the thickness of said deposited amorphous semiconductor material at the gas inlet side (upstream side) as compared to the gas outlet side (downstream side) of the substrate. More particularly, in a continuous deposition apparatus, such as the machine schematically illustrated in FIG. 2, if the amorphous semiconductor material is deposited at a rate of approximately 10 to 12 angstroms per second by increasing the r.f. power utilized to disassociate a two part silane to one part hydrogen gas mixture, an approximately 3500 angstrom thick layer is deposited at the gas inlet side of the substrate as compared to an approximately 6500 angstrom thick layer which is deposited at the gas outlet side thereof.
That which follows is a synopsis of (1) the detailed investigation of the problems of instability, thickness variation and powder formation which occurred at high deposition rates, and (2) the manner in which the upstream cathode assembly of the present invention, briefly discussed hereinbefore, was employed to solve said problem. It will be helpful in understanding the problem and the solution thereof to refer to FIGS. 7-9.
Referring now to FIG. 7, a schematic illustration of the glow discharge plasma deposition region of a deposition chamber is depicted. Basically, precursor reaction gases, such as a one to two ratio of silane and atomic hydrogen, are introduced from the gas manifold to flow around the upstream side of the cathode before entering the deposition plasma region for disassociation and deposition onto the substrate. Spent reaction gases and nodeposited plasma flow around the downstream side of the cathode and are withdrawn from the deposition plasma region through the exit port.
By employing the apparatus schematically represented in FIG. 7, Applicants discovered that powder formation could be eliminated by either (1) increasing the flow rate (volume per unit time) of the reaction gases entering the deposition plasma region, while maintaining the ratio of reaction gas constituents (such as the ratio of silane to molecular hydrogen) substantially constant, or (2) modifying the design of the deposition plasma region so that the cathode to substrate distance "d" is greater than the distance from the wall of the shielding to the edge of the cathode "r" so that gas stagnation does not occur. However, when the thickness of the deposited semiconductor layer was measured, the layer was repeatedly found to be as much as 40-50 percent thinner at the gas inlet side of the substrate than at the gas outlet side thereof. This is shown in FIG. 8 which depicts the thickness of the semiconductor material deposited at the inlet side (t.sub.1) as being typically about 3500 angstroms and the thickness of the semiconductor material deposited at the outlet side (t.sub.2) as being typically 6500 angstroms thick.
Turning now to FIG. 9, a schematic representation of the glow discharge deposition plasma region of a deposition chamber, similar to the representation of FIG. 7, illustrates the manner in which Applicants next attempted to solve the problems of stability, thickness, uniformity and powder free deposition. Previously, the substrate to cathode distance "d" was maintained constant along the entire length of the deposition plasma region. With that in mind, it was hypothesized that with all other parameters remaining constant, by moving the substrate closer to the cathode, the effects of the electromagnetic field developed therebetween would be intensified and the deposition rate of amorphous semiconductor material adjacent the more closely spaced substrate-cathode regions would increase. Accordingly, Applicants attempted to increase the thickness t.sub.1, at the gas inlet side of the substrate relative to the thickness t.sub.2 at the gas outlet side thereof by angling the substrate relative to the cathode in the manner noted by the substrate S.sub.1, shown in phantom outline in FIG. 9. However, it was found that rather than increasing the thickness t.sub.1 of the deposited semiconductor material at the gas inlet side of the substrate, the decrease in substrate to cathode distance resulted in a further decrease in thickness of that deposited material. This was attributed to the "nozzle effect" which the angled substrate produced, said nozzle effect serving to increase the velocity of the reaction gases flowing through the deposition plasma region.
Applicants, while still attempting to solve the aforementioned problems, next decided to increase the substrate to cathode distance in the manner depicted by the substrate S.sub.2 again shown in phantom outline FIG. 9, i.e. by increasing the substrate to cathode distance at the gas inlet side thereof. It was theorized that this design would cause the reaction gases to stagnate adjacent the upstream side of the substrate, by forming a greater upstream substrate to cathode distance than downstream substrate to cathode distance, the thickness (t.sub.1) of deposited semiconductor material at the upstream side of the substrate would increase. While the thickness of the deposited material at the upstream side did increase, and despite Applicants' attempts to vary the degree of angulation of the substrate relative to the cathode, the thickness of the deposited material at the gas inlet side of the substrate could only be increased to a value of approximately 65 percent of the thickness at the gas outlet side.
The foregoing experimental results led Applicants to the conclusion that the reactivity of the reaction gas mixtures travelling through the deposition plasma region was changing as that mixture progressed through said region due to the length of time it was subjected to the effects of the electromagnetic field. It was therefore hypothesized that the reactivity of the reaction gas mixtures at the upstream side of the cathode is much different than the reactivity of the mixtures as the downstream side. This conclusion conforms with other of Applicants' experimental results, mentioned hereinabove, which demonstrate the quality (the optical, electrical and chemical properties) of the amorphous semiconductor material deposited at the upstream side of the cathode is inferior to the quality of the material deposited at the downstream side.
FIG. 10 schematically illustrates the final design of the novel cathode assembly of the present invention which Applicants originated to help to provide a stable, powder-free and uniform thickness of amorphous semiconductor material across the entire length of the substrate. More specifically, the design utilized the concept of flowing the reaction gas mixtures through a predeposition electromagnetic field to begin the disassociation and species recombination thereof. Thus, by the time the now ionized plasma reached the deposition plasma region, the reactivity of the species contained therein was more homogeneous and a powder-free, uniform deposition of semiconductor resulted. Of course, it was necessary to provide a nondepletable supply of process gases and prevent stagnation of those gases as they travelled about the cathode in order for the cathode assembly of the present invention, illustrated in FIG. 10, to achieve said powder-free, uniform deposition.
Applicants have thus solved the powder formation and thickness uniformity problems by utilizing (1) the predeposition technology described supra, to design a special glow discharge deposition plasma region which includes an elongated upstream cathode for subjecting the process gases to a predeposition electromagnetic field prior to introducing those process gases, albeit in ionized form, into the deposition plasma region; (2) the experimental results, described supra, to provide a sufficient flow rate of reaction gas mixtures so as to prevent depletion of those mixtures as they travel through the plasma region; and (3) the experimental results, also described supra, to provide a path of travel for the reaction gas mixtures of sufficient cross-sectional area so as to prevent nonuniform stagnation and compression of those mixtures travelling through the predeposition plasma and deposition plasma regions. The result is a high speed deposition system in which a stable, uniform layer (uniform in electrical, chemical and optical characteristics) of powder-free amorphous semiconductor material can be deposited.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which encompass relatively large areas and which can be doped to form p-type and n-type materials for the production of p-i-n type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous semiconductor alloys by glow discharge deposition techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. One such technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states in the band gaps thereof and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structure therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (V.sub.oc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage may be added, thereby making the greatest use of light energy passing through the cells.
It is of obvious commercial importance to be able to mass produce photovoltaic devices by a continuous process. Unlike crystalline semiconductor materials which are limited to batch processing for the manufacture of solar cells, amorphous semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in U.S. Pat. No. 4,400,409, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; and U.S. patent applications: Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Materials; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in this latter patent application, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a solar cell of a p-i-n type configuration, the first chamber is dedicated for depositing a p-type semiconductor layer, the second chamber is dedicated for depositing an intrinsic semiconductor layer and the third chamber is dedicated for depositing an n-type semiconductor layer.
Whereas, for purposes of mass production, the succession of deposition chambers described hereinabove, is most advantageously employed, a batch processing system may also be used. In such a batch processing system the amorphous semiconductor alloy material can also be deposited in multiple layers over large area substrates to form photovoltaic devices. Batch processing techniques for producing p-i-n type solar cells may proceed in either of two possible manners: (1) a plurality of interlocked deposition chambers are provided wherein a first chamber deposits a p-type semiconductor layer; a second chamber deposits an intrinsic semiconductor layer; and a third chamber deposits an n-type semiconductor layer; or (2) a single deposition chamber is provided which is flushed after the deposition of each p, i, n semiconductor layer. In either case, the batch process techniques are accomplished on individual substrate plates in an intermittent mode of operation.
While both systems, batch and continuous, have their own set of operating problems, they both must be kept free of contaminants, which, if deposited with the semiconductor material onto the deposition surface of the substrate, would harm if not destroy the efficiency and operation of photovoltaic devices produced therefrom. Accordingly, each system must be careful to control the interior environment of its deposition chamber to prevent the influx of contaminants from external sources. After being exposed to the environment, the chambers are pumped, heated and cleansed in an attempt to remove contaminants such as water vapor from the chamber walls. Further, only the purest process gases are purchased for introduction into the chamber and subsequent deposition onto the substrate surface as semiconductor layers. And finally, both systems produce said semiconductor layers by employing very similar operating parameters such as r.f. or microwave power, pressure, process gas mixture, flow rate, temperature, etc.
It should therefore be obvious to those ordinarily skilled in the art that the upstream cathoe system of the present invention is equally well-suited for use with batch processing and continuous production apparatus. With both sets of apparatus, it serves the identical function of creating an electromagnetic field upstream of the deposition cathode for (1) collecting impurities from the process gases and contaminants from the walls of the deposition chambers, and (2) initiating the disassociation of process gases into electrically, chemically and optically superior species which, when deposited onto the substrate, are of substantially homogeneous chemical composition.
It should further be apparent that the upstream cathode assembly, described herein, has great utility in not only (1) increasing the quality and stability of semiconductor material, but also in (2) increasing the rate of deposition of that semiconductor material without inducing powder formation, and (3) while maintaining a constant thickness of that material across the entire surface of the substrate.
These and other objects and advantages of the present invention will become clear from the drawings, the claims and the description of the invention which follow.